In the case where a metal pipe for conveying water, such as a cast iron pipe, is to be branched by a saddled water tap, the drilling of a hole in said pipe is inevitable. Such drilling results in the formation of an exposed region in the metal surface of the pipe, and since the exposed region is subjected to water flowing through the pipe, corrosion takes place, leading to the formation of "red water (containing rust)" in the terminal end of the pipe. Further, there is a danger of a corrosion product being formed in the exposed region and in the worst case, such corrosion product can close the water tapping passage in the water tap.
As a countermeasure, metal (copper) sleeves which can be inserted into drilled holes have already been marketed. However, since such metal sleeve is formed to have a smaller diameter than that of the associated drilled hole, the inner surface of the drilled hole is still subjected to running water; thus, there is almost no corrosion-preventive effect. Further, depending upon the kind of the material of the metal pipe, there is formed a cell owing to the difference in potential between the copper of the metal sleeve and the pipe, said cell accelerating corrosion.
Further, there is known a metal sleeve of copper adapted to be intimately contacted with the inner surface of the associated drilled hole, but even this arrangement tends to form a cell, leading to corrosion.
Thus, the known technical means described above are still insufficient for prevention of corrosion.